It is known that the field of 3D printing by photo-curing can comprise two basic technologies: stereolithographic printing, in which a laser emitting light at around 400 nm is used, to solidify by means of the beam emitted, a photo-curing liquid polymer which is contained in a special tank; and DLP (Digital Light Processing) printing, according to which a photo-curing liquid polymer, again in a tank, is exposed to luminous radiation emitted by a device similar to a projector. According to both these technologies, the printing process proceeds by making one layer of an object after another; that is, solidifying a first layer adhering to a supporting plate (or extraction plate), and then a second layer adhering to the first layer, and so on, until formation of the complete object. Therefore, according to these technologies, the data representing the three-dimensional object to be formed are organised as a series two-dimensional layers which represent transversal sections of the object under construction.
According to the bottom-up method, which may involve machines of either of the SLA and DLP types, the plate for extracting the object moves from the bottom of the tank upwards, as the object is formed layer-by-layer. The basic bottom-up method comprises:                a. a 3D model of the object to be constructed is represented in computer software as an ordered succession of layers, with thickness determined according to the technology adopted, the opacity of the liquid polymer, the quantity of catalyst used with the polymer, the degree of precision to be obtained, and the characteristics of the machine provided, usually said thicknesses being between 50 and 200 microns, but in any case, as a succession of a discrete and finite number of layers;        b. an extraction plate, consisting of a material which is able to facilitate the adhering on itself of the first layer of polymer, moves to a predetermined distance from the first layer and waits for the light beam (SLA or DLP) to solidify the first layer; it then raises by a distance sufficient for the layer just formed to detach from the base of the tank (usually approx. 1 mm) and then lowers by the same distance, less the predetermined distance for the formation of the second layer, and so on until the entire object is formed.        
An improvement of the basic bottom-up method is described in the present Applicant's International Application No. PCT/IT2016/000225, filed Oct. 3, 2016, which claims priority to IT102015000057527, filed Oct. 2, 2015. In that patent application, a self-lubricating substratum (or membrane), which is transparent to the electromagnetic spectrum radiation used to cure the liquid polymer and which is present between the bottom of the tank in which the curing takes place and the layer of the object being formed, is described. The membrane gradually releases a layer of lubricating material which allows the liquid polymer suspended above the membrane to cure thereon, thus reducing suction effects and adherences between the object layer being formed and the tank base. The light source, for example a DLP projector, has an image focus plane at the interface of membrane with the liquid polymer. Photo-curing of the liquid polymer takes place at this interface, according to a projected image. Growth of the object under construction is thereby facilitated.
While such arrangements allow for acceptable printing speeds, they provide only limited x-y resolution (e.g., on the order of 75 microns) in the object under construction. Further, because of the need for significant light intensities to cure the liquid polymer, the DLP projector must be positioned close to the focal plane, limiting the area over which the liquid polymer curing can be effected (e.g., to on the order of 120×90 mm). This proximity of the DLP projector to the work space also introduces optical aberrations and distortions in the focal plane.